1. Field of the Invention
The present invention relates to a condenser microphone, and in particular relates to the structure of a power supply battery housing part and more specifically to the structure of an electrode.
2. Related Background of the Invention
Since the impedance of a microphone unit is extremely high, a condenser microphone incorporates therein an impedance converter comprised of an FET (field effect transistor) as a main body, thereby reducing the output impedance. A power supply is required for driving the impedance converter. The power supply includes a phantom power supply, which is an external power supply, and a built-in power supply comprised of a battery, which is housed in a battery compartment within the microphone.
The sound signal converted by the condenser microphone unit is impedance-converted by the impedance converter and then passes through a power module part that includes a low cut circuit, an output circuit, and the like, to be output as a microphone output. This microphone output is input into an external circuit via an output cable comprised of a balanced shielded cable. The output cable is constructed so as to be removably connected to the microphone body by a standardized three-pin type microphone connector, e.g., a connector defined by EIAJ RC-5236 “Latch-Lock Type Round Connector for Acoustic Equipment”. The three-pin type microphone connector is commonly used with a first pin as ground, a second pin as the hot side of a signal and a third pin as the cold side of a signal. The connector of the microphone body is a male connector, the cable side connector is a female connector, and the both connectors are engaged so that the microphone body is electrically connected to the output cable. To the first pin of the cable side connector a connection end which is an extended part of a shield outer jacket of the microphone cable is connected, and two core wires of the microphone cable are connected to the second pin and third pin, respectively.
If the microphone body or the output cable is irradiated with strong electromagnetic waves and then the electromagnetic waves directly penetrate into the microphone body, or travel through the output cable and penetrate into the microphone body via the connector, these are demodulated by the impedance converter or other circuitry to be output from the microphone as audio frequency noise. In particular, as in recent years, as mobile phones have spread widely, a mobile phone is often used in the vicinity of a microphone and electric waves of the mobile phone more often penetrate into the microphone, thus presenting a serious problem of noise generation caused by high frequencies used in the mobile phone.
The condenser microphone includes a battery compartment for a built-in power supply as described above, and the existence of this battery compartment causes penetration of high frequency electromagnetic waves from the outside. Hereinafter, the reason will be described with reference to an example of a conventional condenser microphone shown in FIG. 3A and FIG. 3B.
In FIG. 3A and FIG. 3B, a body case 10 of a microphone is an approximately cylindrical member, and a part of the peripheral wall of an intermediate portion in the axial direction of the cylinder is removed to form a partially cylindrical shape. In the body case 10, end plates 14 and 16 serving also as an electrode receptacle are fixed to both ends of the portion formed into the partially cylindrical shape, respectively, and an interior space of the partially cylindrical shape of the body case 10 partitioned by the end plates 14 and 16 serves as a battery compartment 12. The battery compartment 12 is capable of housing a dry cell used for the built-in power supply, e.g., one AA dry cell 20. To the end plate 16, an electrode plate 17 formed so as to receive the positive electrode of the dry cell 20 is mounted, and to the other end plate 14, there is mounted a coil spring electrode 18 for pressing the dry cell 20 toward the electrode plate 17, the coil spring electrode 18 being in contact with the negative electrode of the dry cell 20. The outer diameter of the coil spring electrode 18 decreases sequentially from the base toward the tip, the coil spring electrode 18 is allowed to sink into one plane when being pushed by the negative electrode of the dry cell 20, and thereby a large return stroke can be obtained.
The tip (at the left end in FIG. 3A and FIG. 3B) side of the body case 10 partitioned by the end plate 16 is a microphone unit incorporating part 30, and into this microphone unit incorporating part 30 a non-illustrated condenser microphone unit is to be incorporated. The inner periphery of one end of a connector sleeve 28 is fitted around the outer periphery of the back end side of the body case 10, resulting in the connector sleeve 28 being added to the body case 10. At the back end side of the body case 10, a microphone side output connector 40 comprised of a male connector is incorporated into the connector sleeve 28. The output connector 40 includes, behind the end plate 14, a connector base 24 fitted into and fixed to the connector sleeve 28, and connector pins 26 passing through this connector base 24 and being fixed thereto. The output connector 40 is the above-described standardized three-pin type connector, so there are three connector pins 26, however, two pins are illustrated in FIG. 3A and FIG. 3B and the rest pin is hidden behind the two pins. Each connector pin 26 extends in parallel with the axis of the body case 10 and connector sleeve 28. A female-type cable side connector provided at one end of a non-illustrated output cable is coupled to the output connector 40. The cable side connector is fitted into the output connector 40 along the inner peripheral face of the connector sleeve 28, each connector pin 26 of the output connector 40 fits into each receptacle hole of the cable side connector, and thereby the microphone is electrically connected to an external circuit via the output cable.
A cylindrical cover 22 is fitted around the outer periphery of the connector sleeve 28. The cover 22 is movable in the axis direction of the body case 10 while sliding along the outer peripheral face of the connector sleeve 28, and as shown in FIG. 3A, the battery compartment 12 is opened by sliding the cover 22 to the back end side of the body case 10, and as shown in FIG. 3B, by sliding the cover 22 toward the front end side of the body case 10, most of the body case 10 is covered so as to close the battery compartment 12. As shown in FIG. 3A, while the battery compartment 12 is opened, the dry cell 20 can be inserted and removed. As shown in FIG. 3B, the cover 22 covering most of the body case 10 functions also as a grip of the microphone.
As apparent from the above description, the output connector 40 of the microphone exists close to the battery compartment 12, and to this output connector 40 the output cable is connected. Since the battery compartment 12 is constructed so as to be opened and closed with the cover 22 in order to insert and remove a battery and the output connector 40 is constructed so as to insert and remove the cable side connector, there is a gap required for opening and closing or inserting and removing in the battery compartment 12 as well as in the output connector 40. The existence of this gap causes the penetration of high frequency electromagnetic waves into the microphone as described above.
Moreover, the dry cell 20 which is the built-in power supply is grouped into size D, size C, and size AA, however, for example, even with the same AA type, the size thereof differs depending on the manufacturer or the kind of cells. Accordingly, the battery compartment 12 is designed assuming the maximum size among these, and electrical connection needs to be maintained surely even if a dry cell of the smallest size is inserted. Then, the electrode which the negative electrode of the dry cell 20 contacts with is designed so as to obtain a large return stroke as the coil spring electrode 18. In addition to this, the output connector 40 is disposed adjacent to the battery compartment 12, and there is a gap around this output connector 40 as described above, thus providing a portion with poor shielding against the electromagnetic waves. For this reason, the coil spring electrode 18 positioned in the vicinity of the output connector 40 acts as a coil, and the electromagnetic waves that penetrated from the periphery of the output connector 40 are captured by the coil spring electrode 18 and are detected by circuitry in the microphone and output as a noise.
Furthermore, there is also a drawback in case of using a microphone by hand, where a shock is often applied to the microphone due to various causes, such as hitting the microphone against something or dropping it, and the coil spring electrode 18 vibrates every time, thereby causing mechanical noise.
Various kinds of improvements for preventing the mechanical noise of the coil spring electrode have been proposed. For example, stuffing the interior of the coil spring electrode with sponge for restraining the vibration is carried out.
Moreover, a structure is proposed in which a free end of a coil spring electrode is folded back toward the base end side, and while a battery is not inserted in a battery compartment, the tip of the fold-part is in contact with a substrate of the battery compartment by a biasing force of the coil spring electrode, thereby preventing a resonance vibration of the coil spring electrode (e.g., see Patent Document 1).
Furthermore, a structure is proposed in which a bulging part is provided at one end side of a battery compartment so that the coil spring electrode can be forcibly fixed without generating a gap between a holding part that is provided at the one end side of the battery compartment in order to hold the coil spring electrode, and the base of the coil spring electrode held by this holding part (e.g., see Patent Document 2).    [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-178735    [Patent Document 2] Japanese Patent Application Laid-Open No. 58-130362